Inertial terms to magnetization dynamics in ferromagnetic thin films

Inertial magnetization dynamics have been predicted at ultrahigh speeds, or frequencies approaching the energy relaxation scale of electrons, in ferromagnetic metals. Here we identify inertial terms to magnetization dynamics in thin Ni$_{79}$Fe$_{21}$ and Co films near room temperature. Effective magnetic fields measured in high-frequency ferromagnetic resonance (115-345 GHz) show an additional stiffening term which is quadratic in frequency and $\sim$ 80 mT at the high frequency limit of our experiment. Our results extend understanding of magnetization dynamics at sub-picosecond time scales.Comment: 11 pages, 3 figure

Detection of spin torque magnetization dynamics through low frequency noise

We present a comparative study of high frequency dynamics and low frequency noise in elliptical magnetic tunnel junctions with lateral dimensions under 100 nm presenting current-switching phenomena. The analysis of the high frequency oscillation modes with respect to the current reveals the onset of a steady-state precession regime for negative bias currents above $J=10^7 A/cm^2$, when the magnetic field is applied along the easy axis of magnetization. By the study of low frequency noise for the same samples, we demonstrate the direct link between changes in the oscillation modes with the applied current and the normalised low frequency (1/f) noise as a function of the bias current. These findings prove that low frequency noise studies could be a simple and powerful technique to investigate spin-torque based magnetization dynamics

Etude d'un auto-oscillateur non-isochrone (Application à la dynamique non-linéaire de l'aimantation induite par transfert de spin)

Les oscillateurs à transfert de spin (STO) sont des oscillateurs Radiofréquence nanométriques dont la fréquence peut être variée d'un ordre de grandeur. Cette forte agilité en fréquence provient des propriétés non-linéaires de la dynamique de l'aimantation induite par le transfert de spin (STT) dans des multicouches magnétiques nano-structurées. Cette forte agilité en fréquence a le désavantage d'induire une forte sensibilité au bruit. La pureté spectrale des STO est alors bien en dessous des pré-requis pour les applications en télécommunications. Les principales propriétés de la dynamique de l'aimantation induite par le STT ont été décrites simplement à l'aide de la théorie non-linéaire des ondes de spin. Cependant des informations importantes sur le mode d'excitation sont enfouies dans des paramètres phénoménologiques tels que le couplage amplitude-phase NU et le taux de relaxation Gp. La détermination de ces paramètres avec précision est d'un intérêt primordial pour la description de la dynamique non-linéaire. Cette thèse décrit plusieurs méthodes expérimentales pour extraire ces paramètres. La première est la spectroscopie de bruit depuis le domaine temporel qui permet l'extraction des Densités Spectrales de Puissance du bruit d'amplitude et de phase. Leur analyse dans le cadre des modèles théoriques permet non seulement d'extraire directement les paramètres non-linaires mais également de quantifier le bruit de phase qui a un intérêt technologique. Ceci est démontré pour des dispositifs basés sur des jonctions tunnels magnétiques. La deuxième méthode est basée sur l'analyse des largeurs de raies des harmoniques du signal, où il est montré que du fait des propriétés non-isochrones des STO, la relation entre Dfn et Df1 est non triviale et permet l'extraction de NU et Gp. Nous utilisons alors toutes les informations obtenues sur le régime autonome de la dynamique des STO pour comprendre leur dynamique non-autonome qui sont des pré-requis à leurs utilisations dans des architectures RF complexes.Spin Torque Oscillators (STO) are nano-sized Radio-Frequency oscillators whose frequency agility can be tuned by an order of magnitude. This tuning originates from the non-linear properties of the underlying magnetization dynamics that is induced by spin transfer torque (STT) in multilayered magnetic nanostructures. Being highly tunable in frequency has the inconvenient of creating a very strong sensitivity to noise. As a result the spectral purity of STOs is far below the one required for applications for instance in telecommunications. The magnetization dynamics induced by STT has been described theoretically in the frame of nonlinear spin wave theory that makes the essential features of the underlying properties very transparent. However important information on the excitation mode are "buried" in phenomenological parameters such as NU the amplitude-phase coupling and Gp the amplitude relaxation rate. Determining these parameters with accuracy from experiments is thus an important issue. This thesis describes several experimental methods to extract these parameters. The first is time domain noise spectroscopy which permits to extract phase and amplitude noise Power Spectral Densities. Their analysis in the frame of theoretical models allows direct extraction of the nonlinear parameters, but also to quantify the technological relevant phase noise. This is demonstrated for magnetic tunnel junction devices. A second method is the analysis of higher harmonics linewidth, where it is shown that due to the non-isochronous property of STOs, the relationship between Dfn and Df1 is non-trivial and allows to extract NU and Gp. We then apply the information gathered on the autonomous dynamics of STOs to understand the non-autonomous dynamics of STOs that are a prerequisite for the use of STOs in complex RF architectures. It is shown experimentally how the nonlinear parameters influence this non-autonomous behaviour.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Spiking Dynamics in Dual Free Layer Perpendicular Magnetic Tunnel Junctions

Spintronic devices have recently attracted a lot of attention in the field of unconventional computing due to their non-volatility for short and long term memory, non-linear fast response and relatively small footprint. Here we report how voltage driven magnetization dynamics of dual free layer perpendicular magnetic tunnel junctions enable to emulate spiking neurons in hardware. The output spiking rate was controlled by varying the dc bias voltage across the device. The field-free operation of this two terminal device and its robustness against an externally applied magnetic field make it a suitable candidate to mimic neuron response in a dense Neural Network (NN). The small energy consumption of the device (4-16 pJ/spike) and its scalability are important benefits for embedded applications. This compact perpendicular magnetic tunnel junction structure could finally bring spiking neural networks (SNN) to sub-100nm size elements

Coherent long-range transfer of angular momentum between magnon Kittel modes by phonons

We report ferromagnetic resonance in the normal configuration of an electrically insulating magnetic bilayer consisting of two yttrium iron garnet (YIG) films epitaxially grown on both sides of a 0.5-mm-thick nonmagnetic gadolinium gallium garnet (GGG) slab. An interference pattern is observed and it is explained as the strong coupling of the magnetization dynamics of the two YIG layers either in phase or out of phase by the standing transverse sound waves, which are excited through a magnetoelastic interaction. This coherent mediation of angular momentum by circularly polarized phonons through a nonmagnetic material over macroscopic distances can be useful for future information technologies

Unbiased Random Number Generation using Injection-Locked Spin-Torque Nano-Oscillators

Unbiased sources of true randomness are critical for the successful deployment of stochastic unconventional computing schemes and encryption applications in hardware. Leveraging nanoscale thermal magnetization fluctuations provides an efficient and almost cost-free means of generating truly random bitstreams, distinguishing them from predictable pseudo-random sequences. However, existing approaches that aim to achieve randomness often suffer from bias, leading to significant deviations from equal fractions of 0 and 1 in the bitstreams and compromising their inherent unpredictability. This study presents a hardware approach that capitalizes on the intrinsic balance of phase noise in an oscillator injection locked at twice its natural frequency, leveraging the stability of this naturally balanced physical system. We demonstrate the successful generation of unbiased and truly random bitstreams through extensive experimentation. Our numerical simulations exhibit excellent agreement with the experimental results, confirming the robustness and viability of our approach.Comment: 13 pages, 8 figure

Dynamique de l'aimantation de nano-oscillateurs micro-ondes à transfert de spin

Cette thèse s inscrit dans la thématique de l électronique de spin et concerne plus particulièrement la dynamique radiofréquence de l aimantation sous courant polarisé en spin. Dans les nanostructures magnétiques, le transfert de spin permet de soutenir une oscillation entretenue de l aimantation à grande amplitude. Ces oscillations suscitent un intérêt fondamental pour l étude de la dynamique de l aimantation dans le régime fortement non-linéaire. Les oscillateurs à transfert de spin sont également très prometteurs d un point de vue applicatif, du fait de leur taille nanométrique et de leur forte agilité en fréquence. Néanmoins, leur signal de sortie devra être amélioré et l origine de leur pureté spectrale comprise. Dans cette optique, nous avons étudié expérimentalement la réponse dynamique de deux types d oscillateur complémentaires. Nous nous sommes d abord intéressés à une structure vanne de spin basée sur une couche polarisante à aimantation perpendiculaire. Nous avons démontré la possibilité d induire une précession de l aimantation de forte amplitude autour de son maximum d énergie. Le renfort de simulations micromagnétiques fut nécessaire pour décrire précisément les observations expérimentales. Dans un second temps, nous avons développé un banc de mesure temporelle ainsi qu un protocole d analyse original pour étudier la pureté spectrale d oscillateurs à jonction tunnel magnétique MgO. Ces mesures nous ont permis d observer certains des mécanismes d instabilité limitant la cohérence du signal micro-onde de l oscillateur, comme par exemple des fluctuations de fréquence sur l échelle de la dizaine de nanosecondes.This thesis addresses a current topie of spinelectronics which are steady-state oscillations of the magnetization induced by spin transfer torque. ln magneticl nanostructures, the large oscillation amplitude is of interest since it allows probing of the magnetization dynamics in the non-linear regime but also fo~ integrated microwave components. Here the small size of the devices and the relatively large frequency tuning range are promising properties for futurel telecommunications. However, before integration major issues need to be addressed such as increasing the output power and understanding the spectral purity. Therefore, we studied experimentally the dynamic behaviour of two types of spin transfer oscillators. The fll"st type focuses on a spin valve structure with a perpendicularly magnetized polarizing layer. We show that large amplitude magnetization oscillations around its energy maximum can be established as a resuh of the perpendicular polarizer. With the help of micromagnetic simulations, we were able to interpret qualitatively the experimental resuhs. The second type of structures are MgO based magnetic tunnel junction oscillators of large output power. We developed an experimental set-up to follow the magnetization dynamics in the time domain as well as a data processing protocol to analyze the spectral purity. We identified different transient processes limiting the coherence of the magnetic oscillations, in particular frequency fluctuations on tens of nanoseconds time-scale.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF